Freeze drying or lyophilization is a well-known technique for preserving foods for storage at ambient temperatures for extended periods of time, provided the dried foods are maintained at very low moisture levels during storage. When properly prepared and stored, such freeze-dried foods may be rehydrated when desired for consumption. Depending on the freeze-drying technique employed and how well the foods were stored, the reconstituted foods may be of quality approximating that of freshly prepared foods, or may be of significantly lower quality (i.e., the reconstituted foods may be mushy, rubbery, or otherwise unappealing).
In recent years, the armed forces have found compact freeze-dried foods desirable as meals ready to eat (MREs), as they are both space and weight efficient for storage on vehicles (particularly submarines and aircraft) and are easy for the average infantryman to carry. Further, compact and light freeze-dried foods are attractive for long term storage in emergency safety facilities and as emergency meals for sudden deployment to disaster victims. The freeze-drying process typically involves substantially removing the water from the food products by freezing them and then reducing the surrounding air pressure while adding sufficient heat to allow the frozen water to sublime directly from solid to gaseous phase. There are typically three stages in the freeze-drying process. First, the material to be dried is frozen to a temperature below that of the formation of water ice, and, more typically, to below the eutectic point of water (the lowest temperature where solid water ice and liquid water can coexist). This ensures that sublimation rather than melting will occur during the subsequent water removal steps. During the next phase the pressure is lowered and enough heat is supplied to the frozen food for the frozen water to sublimate. In this step, most of the water is removed via sublimation. Often, sublimation occurs under a partial vacuum to speed the drying process. Finally, a secondary drying step is often employed to remove adsorbed water accumulated during the preceding steps. In this phase, the temperature is raised even higher than in the primary drying phase to break any physico-chemical interactions that have formed between the water molecules and the frozen food. Typically, the pressure is also lowered in this stage to encourage sublimation, but the pressure may be elevated as well. After the freeze drying process is complete, the vacuum is usually broken with an inert gas, such as nitrogen, before the material is sealed.
If a freeze-dried food is adequately sealed to prevent the reintroduction of moisture, the freeze-dried food may be stored indefinitely at room temperature without spoilage. Such storage is possible because the low moisture content inhibits the action of bacteria and enzymes that would otherwise act to spoil or degrade the substance.
The freeze-drying process does not result in significant shrinkage of the freeze-dried foods. Thus, once freeze-dried, the food may be compressed or compacted for more efficient storage. One commonly employed method of compacting freeze-dried foods involves, after freeze-drying, spraying the food with sufficient water to raise its average moisture content to between about 5 and about 13% and then compressing the dried food. The moisture level is increased to make the food more plastic to allow the food to flow instead of shatter during compaction and to allow the food to retain its cellular structure. However, to assure adequate and homogeneous plasticization of the freeze-dried food, the dried food is sprayed with water (or, more typically, an aqueous solution of a gum or the like) and is then given sufficient time for the water to substantially equilibrate throughout the food. This process of homogenization may take hours or even days to complete. Further, after compaction, the added moisture must once again be removed via a dehydration step if the food is to benefit from the drying process enough to be stored at room temperature, again adding to the time and expense involved.
It is desirable to be able to achieve the freeze-drying and compaction of the food mass without the lengthy rehydration process. However, if the food mass is only partially freeze-dried to an average moisture content of 5 to 13 percent, the moisture distribution is typically uneven, with the exterior of the food mass being almost completely dry and the core being still full of ice to almost it's normal moisture content. In one compaction method, the food mass is partially freeze-dried and then microwave heated to assist in melting the core to more rapidly remove the core water and achieve the level of hydration commensurate with the plasticity of the food mass for compaction. The food is then compacted and then dried sufficiently for storage.
In another technique, the food mass is first partially dehydrated, and then freeze dried to moisture content level sufficient for plasticity. The food mass is then compacted, and then freeze-dried for storage.
Once freeze-dried and compacted, the food may be stored for long, extended periods at room temperature. The storage time is limited by the how well the packaging of the food keeps out moisture. Most freeze-dried foods are sealed in plastic or metal containers, and may be so preserved for years or even decades. However, metal and plastic seals are still slightly porous and may degrade over time such that their porosity gradually increases, yielding an upper safe storage limit of about 25-30 years. While this is more than sufficient for most applications, food prepared and stored for use in rare and unusual emergency situations may be required to be stored for 50 to 60 years or even longer. Thus, there remains a need for an improved system for storing freeze-dried foods. The present novel technology addresses this need.